1. Field of the Invention
This invention relates to a process for upgrading hydrocarbons of relatively low boiling point and relatively poor antiknock properties into hydrocarbons having higher antiknock ratings and suitable for use as motor fuel blending stocks. More particularly, the invention relates to the use of a catalyst system comprising trifluoromethanesulfonic acid and hydrogen fluoride in conjunction with a Lewis acid for the conversion of natural gasoline derived from natural gas into a gasoline product of higher octane number than the original charge stock.
2. Description of the Prior Art
Saturated highly branched-chain paraffin hydrocarbons, and particularly mixtures thereof, occurring in gasoline fractions are very desirable motor fuel components because of their antiknock properties. It is known, for example, that the more highly branched chain isomers of paraffins are more valuable than the corresponding unbranched or less branched paraffins because of their higher octane rating. The 2,2-dimethylbutane isomer of hexane, for example, has an octane number of 92 whereas normal hexane has an octane rating of 25. Similarly, 2,2,3-trimethylbutane (triptane) also has a highly branched chain structure and provides fuel characteristics far exceeding the isoheptanes such as methylhexanes and dimethylpentanes. Because of its highly branched structure, the octane rating of triptane exceeds that of isooctane which has a rating of 100. The demand for motor fuels of high octane value without lead additives has increased steadily and a variety of hydrocarbon processes have been suggested whereby various hydrocarbon oils, such as straight run gasolines, naphthas and similar hydrocarbon mixtures, have been upgraded to produce gasoline of higher octane value.
U.S. Pat. No. 3,142,633 relates to conversion of naphthas to hydrocarbons boiling in the middle distillate range by treatment with anhydrous liquid HF, either alone or augmented with up to 60 mol percent of BF.sub.3.
U.S. Pat. No. 3,594,445 discloses the isomerization of normal and naphthenic paraffins by employing a catalyst containing (1) a fluoride of a metal of Group V such as antimony pentafluoride and (2) a fluorosulfonic acid in the presence of hydrogen and an olefin and/or an alkyl fluorosulfonate.
U.S. Pat. No. 3,766,286 discloses the isomerization of paraffinic and/or alkyl substituted aromatic hydrocarbons with a catalyst comprising (a) a Lewis acid of the formula MX.sub.n where M is selected from Group IV-B, V or VI-B of the Periodic Table, X is a halogen and n varies from 3 to 6, and (b) a strong Bronsted acid comprising fluorosulfuric acid, trifluoromethanesulfonic acid, trifluoroacetic acid or mixtures thereof.
U.S. Pat. No. 3,839,489 discloses the isomerization of paraffinic feedstocks using a catalyst consisting of arsenic pentafluoride, antimony pentafluoride, and mixtures thereof with either trifluoromethanesulfonic acid or hydrogen in the presence of hydrogen.
U.S. Pat. No. 3,996,116 discloses the conversion of straight-chain saturated hydrocarbons into branched-chain saturated hydrocarbons by oxidizing the hydrocarbons in liquid phase in the presence of a superacid such as HFSO.sub.3 by electrolysis at a voltage between the half wave voltage of the hydrocarbon and that of the superacid.
U.S. Pat. No. 4,044,069 discloses a process for the isomerization and alkylation of paraffinic hydrocarbons by means of a catalyst composition comprising a mixture of Lewis acids such as arsenic pentafluoride and a sulphonic acid corresponding to the general formula RF (SO.sub.3 H)p where RF represents an alkylperfluorinated or cycloalkylperfluorinated hydrocarbon radical having a number of carbon atoms between 2 and 8 and p is 1 or 2.
Other pertinent literature art includes the articles entitled "Lower Paraffin Hydrocarbons--Catalytic Conversion by BF.sub.3 With HF" by E. C. Hughes and S. M. Darling (Std. Oil Co. 1951) Ind. & Eng. Chemistry, Vol. 43, No. 3, p. 746, and "Mechanism of Acid-Catalyzed Isomerization of the Hexanes" by D. A. McCaulay (Std. Oil Co. 1959) J. Am. Chem. Soc., Vol. 81, p. 6437.
None of the prior art described above discloses a catalyst composition of the type hereinafter described or the use of the same for the direct conversion of natural gasoline in accordance with the invention. Although a number of patents have been granted on processes involving Lewis acid and/or Bronsted catalysis, such processes have generally been based on the isomerization of relatively pure hydrocarbons such as butane, pentane, hexane or heptane and are primarily adapted for such isomerizations. In the patent literature, it is also generally stated that processes for the isomerization of pure hydrocarbons are also applicable to various distillates, including kerosene, natural gasoline, straight run gasoline, and the like. While such processes undoubtedly can be applied to certain distillate fractions, they do not serve as a useful prototype for the catalytic upgrading of distillates of the nature of gasoline due to the fact that octane numbers are only slightly improved and, in many cases, the octane improvement is substantially nil. See, for example, U.S. Pat. No. 2,373,674.